LTC3411 Linear Technology, LTC3411 Datasheet - Page 14

• Manufacturer Part Number: LTC3411
• Description: 1.25A/ 4MHz/ Synchronous Step-Down DC/DC Converter
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
LTC3411
Thermal Considerations
In a majority of applications, the LTC3411 does not
dissipate much heat due to its high efficiency. However, in
applications where the LTC3411 is running at high ambi-
ent temperature with low supply voltage and high duty
cycles, such as in dropout, the heat dissipated may exceed
the maximum junction temperature of the part. If the
junction temperature reaches approximately 150 C, both
power switches will be turned off and the SW node will
become high impedance.
To avoid the LTC3411 from exceeding the maximum
junction temperature, the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated exceeds the
maximum junction temperature of the part. The tempera-
ture rise is given by:
where P
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature, T
As an example, consider the case when the LTC3411 is in
dropout at an input voltage of 3.3V with a load current of
1A. From the Typical Performance Characteristics graph
of Switch Resistance, the R
P-channel switch is 0.11 . Therefore, power dissipated
by the part is:
The MS10 package junction-to-ambient thermal resis-
tance,
Therefore, the junction temperature of the regulator oper-
ating in a 70 C ambient temperature is approximately:
Remembering that the above junction temperature is
obtained from an R
the junction temperature based on a higher R
it increases with temperature. However, we can safely
14
P
T
J
D
= 0.11 • 120 + 70 = 83.2 C
= I
T
D
T
JA
2
RISE
J
is the power dissipated by the regulator and
, will be in the range of 100 C/W to 120 C/W.
• R
= T
DS(ON)
= P
RISE
D
+ T
•
U
DS(ON)
= 110mW
AMBIENT
JA
U
at 25 C, we might recalculate
J
, is given by:
DS(ON)
W
resistance of the
DS(ON)
U
since
JA
assume that the actual junction temperature will not
exceed the absolute maximum junction temperature of
125 C.
Design Example
As a design example, consider using the LTC3411 in a
portable application with a Li-Ion battery. The battery
provides a V
mum of 1A in active mode and 10mA in standby mode. The
output voltage is V
power in standby, Burst Mode operation is selected for
good low load efficiency.
First, calculate the timing resistor:
Use a standard value of 324k. Next, calculate the inductor
value for about 30% ripple current at maximum V
Choosing the closest inductor from a vendor of 2.2 H,
results in a maximum ripple current of:
For cost reasons, a ceramic capacitor will be used. C
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
The closest standard value is 22 F. Since the output
impedance of a Li-Ion battery is very low, C
10 F. In noisy environments, decoupling SV
with an R6/C8 filter of 1 /0.1 F may help, but is typically
not needed.
The output voltage can now be programmed by choosing
the values of R1 and R2. To maintain high efficiency, the
R
L
C
T
I
OUT
L
1
MHz
9 78 10 1
1
.
MHz
2 5
2 5
IN
.
2 5
•
.
•
510
.
= 2.5V to 4.2V. The load requires a maxi-
1
V
• .
MHz
V
2 2
11
mA
OUT
MHz
• ( %• . )
•
•
1
= 2.5V. Since the load still needs
5
A
1
1
2 5
1 08
2 5
4 2
2 5
4 2
.
.
.
.
.
V
V
V
V
V
323 8
20
460
2
.
H
F
k
mA
IN
IN
is typically
from PV
sn3411 3411fs
IN
:
OUT
IN